.alpha.-Alumina powder is widely used as an abrasive, a raw material of sintered products, a plasma spraying material, a filler, etc. .alpha.-Alumina powder obtained by conventional processes comprises irregular-shaped polycrystals, contains many agglomerates, and has a broad particle size distribution. For some uses, the purity of these conventional species is insufficient. In order to eliminate these problems, .alpha.-alumina powder produced by special processes as hereinafter described has been employed for specific uses. However, these special processes still fail to arbitrarily control the shape or particle diameter of .alpha.-alumina. It has thus been difficult to obtain .alpha.-alumina powder having a narrow particle size distribution.
Known special processes for producing .alpha.-alumina powder include a process utilizing a hydrothermal reaction of aluminum hydroxide (hereinafter referred to as hydrothermal treatment process); a process comprising adding a flux to aluminum oxide, fusing, and precipitating (hereinafter referred to as flux process); and a process in which aluminum hydroxide is calcined in the presence of a mineralizer.
With respect to a hydrothermal process, JP-B-57-22886 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process in which corundum is added as a seed crystal to control the particle size. The process consists of synthesis in a high temperature under a high pressure, making the resulting .alpha.-alumina powder expensive.
According to the study by Matsui, et al. (Hydrothermal Hannou (Hydrothermal Reactions), Vol. 2, pp. 71-78 "Growth of Alumina Single Crystal by Hydrothermal Methods"), an .alpha.-alumina single crystal obtained by growth of an alumina single crystal containing chromium on a sapphire (.alpha.-alumina) seed crystal by a hydrothermal growth process (hydrothermal treatment process) contains cracks. On examining the homogeneity of the crystal in an attempt to clarify the cause of the cracks, it was confirmed that a high strain exists in the boundary between the seed crystal and the grown crystal and that the density of etch pit in the grown crystal near the boundary, which seems to correspond to the dislocation density, is high. The report goes that the cracks are expected to relate to such a strain or a defect and that a hydrothermal growth process is apt to involve incorporation of a hydroxyl group or water into crystals, which appears to cause a strain or a defect.
A flux process has been proposed as a means for controlling the shape or particle size of .alpha.-alumina powder for use as an abrasive, a filler, etc. For example, JP-A-3-131517 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process comprising calcining aluminum hydroxide in the presence of a flux with fluorine having a melting point of not more than 800.degree. C. to prepare .alpha.-alumina particles having an average particle size of from 2 to 20 .mu.m and a hexagonal plate shape having a D/H ratio of from 5 to 40, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of .alpha.-alumina, and H represents a diameter perpendicular to the hexagonal lattice plane. However, this process cannot provide fine .alpha.-alumina powder having a particle diameter of not more than 2 .mu.m, and all the particles obtained have a plate shape. Therefore, the resulting .alpha.-alumina powder is not always suitable for use as abrasives, fillers and a raw materials for single crystals.
Several proposals have been made to date as a solution to these problems. JP-A-59-97528 discloses a process for improving the shape of .alpha.-alumina particles comprising calcining aluminum hydroxide prepared by the Bayer process in the presence of boron containing ammonium and a boron mineralizer to obtain .alpha.-alumina powder having an average particle diameter of from 1 to 10 .mu.m and a D/H ratio approximate to 1. However, this process involves problems in that the boron-containing or fluorine-containing material added as a mineralizer remains in the resulting .alpha.-alumina and agglomerates are formed upon calcining.
In connection to calcination of sodium-containing aluminum hydroxide prepared by the Bayer process, it has been proposed to conduct calcining in the presence of a fluoride, e.g., aluminum fluoride or cryolite, and a chlorine-containing compound, e.g., chlorine or hydrogen chloride in British Patent 990,801; or calcining in the presence of boric acid, and ammonium chloride, hydrochloric acid or aluminum chloride in West German Patent 1,767,511, for the purpose of effectively removing sodium while controlling the particle diameter.
However, in the former process, since a mineralizer such as aluminum fluoride is added in a solid form or the calcination is conducted while supplying chlorine gas and fluorine gas without the addition of water, the resulting alumina particles have problems of an irregular shape and a broad particle size distribution. The latter process also involves a problem in that boric acid as a mineralizer remains in the resulting .alpha.-alumina in the form of a boron-containing material. In addition, these processes aim chiefly at removal of sodium, and the sodium salt, such as NaCl or Na.sub.2 SO.sub.4, by-produced by the reaction between sodium and a sodium removing agent must be sublimed or decomposed by calcination at a high temperature of at least 1,200.degree. C.
With respect to the reaction between alumina and hydrogen chloride gas, there is a report in Zeit. fur Anorg. und Allg. Chem., Vol 21, p. 209 (1932) of an equilibrium constant of the reaction system comprising sintered .alpha.-alumina having a particle diameter of from 2 to 3 mm, hydrogen chloride, and produced aluminum chloride. According to the report, while .alpha.-alumina is found produced in a place different from the place where the starting material has been charged, only hexagonal plate-shaped particles are obtained.
JP-B-43-8929 discloses a process comprising calcining alumina hydrate in the presence of ammonium chloride to produce alumina having a low impurity content and an average particle diameter of not more than 10 .mu.m. The resulting alumina powder has a broad particle size distribution.
Therefore, none of the conventional techniques succeeded in providing .alpha.-alumina single crystals and satisfying requirements of purity and structural homogeneity of individual particles.
An object of the present invention is to solve the above problems and to obtain high purity .alpha.-alumina which comprises homogeneous and non-agglomerated .alpha.-alumina single crystal particles from various raw materials. More specifically, it is to provide .alpha.-alumina in the form of powder comprising uniform .alpha.-alumina single crystal particles which have an octa- or higher polyhedral shape, a D/H ratio of from 0.5 to 3.0, a number average particle diameter of more than 5 .mu.m and not more than 30 .mu.m, a narrow particle size distribution, a high alumina purity and uniform composition within the particle, with the individual particles being freed of structural strain.